Case study:River Darent at Hawley Manor

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Location: 51° 25' 30", 0° 13' 51"
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Project overview

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Status Complete
Project web site
Themes Habitat and biodiversity, Hydromorphology
Country England
Main contact forename Tom
Main contact surname Cook
Main contact user ID
Contact organisation Environment Agency
Contact organisation web site http://www.environment-agency.gov.uk/
Partner organisations
Parent multi-site project
This is a parent project
encompassing the following
projects
No
The restored reach of the River Darent at Hawley Manor after two years

Project summary

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The River Darent in the Dartford area has been heavily modified over many years,including changes to channel planform, the implementation of land drainage schemes and abstraction,leading to an over widened channel. Prior to restoration this section of river, two miles upstream of Dartford, was very uniform with a shallow gradient and uniform cross section. The natural substrate is dominated by pebbles and gravel but had become overlain by silt. Flow and habitat diversity was limited with negative impacts on fish and macroinvertebrate communities. The aim of the project was to demonstrate that the processes that sustain a healthy chalk stream could be restored and the habitat protected during drought periods. This was to be achieved in a cost-effective way by re-working the in-channel gravels to form a low flow channel. The restoration work contributed to Chalk Rivers Biodiversity Action Plan (BAP) targets and complemented the implementation of the Darent Action Plan (1992).

All of the bed modifications were carried out from within the channel, so the banks and trees were left untouched. The channel was re-profiled using a long reach excavator creating a sinuous channel, with pool and riffle sequences, still within the confines of the original channel. The excavator accessed the river where there were suitable gaps between trees and the creation of gravel berms at these points enabled the excavator to exit the river without damaging the banks.

A low flow channel was created by moving small amounts of gravel in a meandering path within the confines of the wider channel. Part of the bed was kept at a higher elevation to create a sequence of riffles. In other areas more significant quantities of gravel were redistributed enabling pools to be created. Pools, spaced at approximately 20m intervals, were designed to be self-cleaning. Gravels were placed upstream of each to narrow the flow and increase velocity to induce scour in the pools.

The whole of the construction phase was supervised by the designer who was on site throughout the work and provided instructions to the excavator driver. Existing bankside trees with large root systems acted as natural deflectors and provided a variety of marginal habitat. A small amount of planting was carried out, including water crowfoot which was sourced locally from the Darent.

Monitoring surveys and results

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Lessons learnt

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Image gallery


River Darent at Hawley Manor before works
Immediately before works
During works
Excavator carrying out in-channel works
Immediately after works
The same reach after five years
In-channel vegetation after four years
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Catchment and subcatchment

Catchment

River basin district Thames
River basin Darent

Subcatchment

River name Mid Darent
Area category 100 - 1000 km²
Area (km2)
Maximum altitude category 200 - 500 m
Maximum altitude (m) 244
244 m
0.244 km
24,400 cm
Dominant geology Calcareous
Ecoregion Great Britain
Dominant land cover Arable and Horticulture
Waterbody ID GB106040024222



Other case studies in this subcatchment: Darent Valley Path, Lullingstone Castle, Lullingstone Castle phase 2


Site

Name River Darent at Hawley Manor
WFD water body codes GB106040024222
WFD (national) typology
WFD water body name Mid Darent
Pre-project morphology Over-widened, uniform cross section and shallow gradient
Reference morphology Narrow, sinuous channel with flow variation
Desired post project morphology
Heavily modified water body No
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology Groundwater
Dominant substrate Chalk
River corridor land use Semi urban
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)


Project background

Reach length directly affected (m) 250
250 m
0.25 km
25,000 cm
Project started
Works started 2005/09/24
Works completed 2005/09/24
Project completed 2005/09/25
Total cost category 1 - 10 k€
Total cost (k€) 2,114.00
2,114 k€
2,114,000 €
Benefit to cost ratio
Funding sources Environment Agency

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance
Monitoring



Reasons for river restoration

Mitigation of a pressure Abstraction and land drainage schemes resulting in change to channel planform
Hydromorphology Quantity & dynamics of flow, Substrate conditions
Biology Invertebrates, Macrophytes
Physico-chemical
Other reasons for the project


Measures

Structural measures
Bank/bed modifications Re-working of gravels to create riffles, Creation of pools, Creation of bars
Floodplain / River corridor
Planform / Channel pattern Channel narrowing, Adding sinuosity
Other
Non-structural measures
Management interventions
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative
Substrate conditions Yes Yes Yes No No Improvement

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative
Invertebrates: Taxonomic composition Yes Yes No Yes No Improvement
Macrophytes Yes Yes Yes Yes No Improvement

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Any other monitoring, e.g. social, economic

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional links and references

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Supplementary Information

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